1. Technical Field
This invention relates generally to flat type light source device, widely used for general lighting or illumination, or for back light illuminating passive type display device such as liquid crystal display device. More particularly, this invention relates to flat type light source device utilizing edge light effect in which multiple reflection of light rays are carried out between front and rear surfaces of the light transmission panel.
2. Description of Prior Art
Liquid crystal displays are a typical example of passive type displays which do not emit light by them selves, different from active type displays which emit light by them selves such as CRT and electro-luminescence. Accordingly, the liquid crystal displays of transmissive type instead of reflective type are utilized in combination with the flat type light source or light transmission panel for back light which is located in rear of such display, so that it can be clearly observed by an observer in dark place as well as bright place.
In accordance with the prior art, such light transmission panel is widely used for the flat type light sources as back lights or film viewers to illuminate the passive type displays such as liquid crystal displays of transmissive type or the transparent picture film such as photographic films, by combination with light source such as fluorescent lamp of straight line type, located at side surface of said light transmission panel.
This type of lighting system is called as edge lighting or light transmission panel system, wherein the light transmission panel is made from transparent materials with high light transmission characteristics such as acrylic resin, polycarbonate resin or glass. Light rays emitted from light source are input to side surface of the light transmission panel. Such light rays repeat multiple reflection between front and rear surfaces in the light transmission panel. The light rays leak or output gradually according to transmission of light rays to illuminate an object such as the liquid crystal display or the picture film.
The light transmission panel is ordinarily of panel or film having rectangular illumination surface area substantially corresponding to that of the liquid crystal display.
The light transmission panel for illumination has ordinarily rectangular surfaces with a pair of long sides and a pair of short sides. There are following four types of arrangement according to positioning between the light transmission panel and the line light source such as fluorescent lamp of straight line type;
(1) An arrangement of short side-one light type is such that one line light source is placed at a short side surface of the light transmission panel. PA1 (2) An arrangement of long side-one light type is such that one line light source is placed at a long side surface of the light transmission panel. PA1 (3) An arrangement of short side-two lights type is such that two line light sources are placed at two short side surfaces in the light transmission panel. PA1 (4) An arrangement of long side-two lights type is such that two line light sources are placed at two long side surfaces in the light transmission panel.
As well known in this field, incident light rays into the light transmission panel for illumination are transmitted inside the light transmission panel and the light rays repeat multiple internal reflection from light incident side toward opposite side in the light transmission panel. Some amount of transmitted light rays reach until the opposite side in the light transmission panel. The rest of transmitted light rays leak little by little from surface of the light transmission panel for illumination to outside of the light transmission panel to illuminate the passive type display or the picture film.
Accordingly, in case of the short side-one light type or long side-one light type, brightness of surface projecting from surface of the light transmission panel is decreased extremely according to distance apart from location of the light source.
Such one light types can not obtain uniform surface brightness and have lower surface brightness according to distance apart from location of the light source.
In case the light transmission panel with large surface area is required for illumination, the one light types can not be used because distance to be transmitted is too long and brightness of surface is extremely low, near opposite side terminal far from light input side in the light transmission panel.
Therefore, the one light types are limited in usage for the liquid crystal displays of color indication type with small surface area or that of monochrome indication type not requiring relatively high and uniform surface brightness.
The two lights types are used for the liquid crystal displays of color and monochrome indication types with large surface area requiring relatively high and uniform surface brightness, such as monitors or displays for personal computers or television receivers.
Especially, movable electronic devices driven by battery power source having the liquid crystal display with back light require light weight or compact size, such as for personal computers and information & telecommunication terminals of note or handy size.
The two lights type using fluorescent lamps is not suitable for this purpose, because weight is almost double and housing space to accommodate lamps is almost double compared with one light type.
The fluorescent lamps decrease brightness and uniformity of emission at different surface spots on the lamp surface, because blackened area increases from electrodes toward center of the lamp according to a lapse of time, as known in fluorescent lamps for general lighting purpose. And blackened area is increasing according to a lapse of time and the blackened area differs by each different lamp.
Therefore, in two lights type using fluorescent lamps, uniformity is decreasing in brightness and displayed color at different spots on the surface of light transmission panel according to a lapse of time, compared with one light type.